Copyright 2002 Prentice-Hall, Inc.

Modern Systems Analysisand Design

Third Edition

Jeffrey A. Hoffer Joey F. George

Joseph S. Valacich

Chapter 8Structuring System

Requirements:Process Modeling

8.18.1

Learning Objectives

Understand the logical modeling of processes through studying data flow diagramsHow to draw data flow diagrams using rules and guidelinesHow to decompose data flow diagrams into lower-level diagramsBalancing of data flow diagrams

8.28.2

Learning Objectives

Explain the differences among four types of DFDs: current physical, current logical, new physical and new logicalDiscuss the use of data flow diagrams as analysis toolsCompare and contrast data flow diagrams with Oracle’s process modeling tool and with functional hierarchy diagrams Discuss process modeling for Internet applications

8.38.3

Process Modeling

Graphically represent the processes that capture, manipulate, store and distribute data between a system and its environment and among system componentsData flow diagrams (DFD)� Graphically illustrate movement of data

between external entities and the processes and data stores within a system

8.48.4

Process Modeling

Modeling a system’s process� Utilize information gathered during

requirements determination� Structure of the data is also modeled in

addition to the processesDeliverables and Outcomes� Set of coherent, interrelated data flow

diagrams

8.58.5

Process Modeling

Deliverables and outcomes (continued)� Context data flow diagram (DFD)

� Scope of system� DFDs of current system

� Enables analysts to understand current system� DFDs of new logical system

� Technology independent� Show data flows, structure and functional

requirements of new system

8.68.6

Process Modeling

Deliverables and outcomes (continued)� Project dictionary and CASE repository

8.78.7

Data Flow Diagramming Mechanics

Four symbols are used� See Figure 8-2� Two different standard sets can be used

� DeMarco and Yourdan� Gane and Sarson

8.88.8

Figure 8-2Comparison of DeMarco & Yourdan and Gane &

Sarson DFD symbol sets

8.98.9

Data Flow Diagramming Mechanics

Data Flow� Depicts data that are in motion and moving

as a unit from one place to another in the system.

� Drawn as an arrow� Select a meaningful name to represent the

data

8.108.10

Data Flow Diagramming Mechanics

Data Store� Depicts data at rest� May represent data in

� File folder� Computer-based file� Notebook

� The name of the store as well as the number are recorded in between lines

8.118.11

Data Flow Diagramming Mechanics

Process� Depicts work or action performed on data

so that they are transformed, stored or distributed

� Number of process as well as name are recorded

8.128.12

Data Flow Diagramming Mechanics

Source/Sink� Depicts the origin and/or destination of the

data� Sometimes referred to as an external entity� Drawn as a square symbol� Name states what the external agent is� Because they are external, many

characteristics are not of interest to us

8.138.13

Data Flow Diagramming Definitions

Context Diagram� A data flow diagram (DFD) of the scope of an

organizational system that shows the system boundaries, external entities that interact with the system and the major information flows between the entities and the system

Level-O Diagram� A data flow diagram (DFD) that represents a

system’s major processes, data flows and data stores at a high level of detail

8.148.14

Developing DFDs: An Example

Hoosier Burger’s automated food ordering systemContext Diagram (Figure 8-4) contains no data storesNext step is to expand the context diagram to show the breakdown of processes (Figure 8-5)

8.158.15

Figure 8-4Context diagram of Hoosier Burger’s

food ordering system

8.168.16

Figure 8-5Level-0 DFD of Hoosier Burger’s food ordering system

8.178.17

Data Flow Diagramming Rules

Basic rules that apply to all DFDs� Inputs to a process are always different

than outputs� Objects always have a unique name

� In order to keep the diagram uncluttered, you can repeat data stores and sources/sinks on a diagram

8.188.18

Data Flow Diagramming Rules

Process� No process can have

only outputs (a miracle)

� No process can have only inputs (black hole)

� A process has a verb phrase label

Data Store� Data cannot be moved

directly from one store to another

� Data cannot move directly from an outside source to a data store

� Data cannot move directly from a data store to a data sink

� Data store has a noun phrase label

8.198.19

Data Flow Diagramming Rules

Source/Sink� Data cannot move

directly from a source to a sink

� A source/sink has a noun phrase label

Data Flow� A data flow has only one

direction of flow between symbols

� A fork means that exactly the same data goes from a common location to two or more processes, data stores or sources/sinks

8.208.20

Data Flow Diagramming Rules

Data Flow (Continued)L. A join means that exactly the same data comes

from any two or more different processes, data stores or sources/sinks to a common location

M. A data flow cannot go directly back to the same process it leaves

N. A data flow to a data store means updateO. A data flow from a data store means retrieve or

useP. A data flow has a noun phrase label

8.218.21

Decomposition of DFDs

Functional decomposition� Act of going from one single system to many

component processes� Repetitive procedure� Lowest level is called a primitive DFD

Level-N Diagrams� A DFD that is the result of n nested

decompositions of a series of subprocesses from a process on a level-0 diagram

8.228.22

Balancing DFDs

When decomposing a DFD, you must conserve inputs to and outputs from a process at the next level of decompositionThis is called balancingExample: Hoosier Burgers� In Figure 8-4, notice that there is one input to the

system, the customer order� Three outputs:

� Customer receipt� Food order� Management reports

8.238.23

Balancing DFDs

Example (Continued)� Notice Figure 8-5. We have the same

inputs and outputs� No new inputs or outputs have been

introduced� We can say that the context diagram and

level-0 DFD are balanced

8.248.24

Balancing DFDs

An unbalanced example� Figure 8-10� In context diagram, we have one input to

the system, A and one output, B� Level-0 diagram has one additional data

flow, C� These DFDs are not balanced

8.258.25

Figure 8-10An unbalanced set of data flow diagrams

(a) Context diagram(b) Level-0 diagram

8.268.26

Balancing DFDs

We can split a data flow into separate data flows on a lower level diagram (see Figure 8-11)Balancing leads to four additional advanced rules (See Table 8-3)

8.278.27

Four Different Types of DFDS

Current Physical� Process label includes an identification of

the technology (people or systems) used to process the data

� Data flows and data stores are labeled with the actual name of the physical media on which data flow or in which data are stored

8.288.28

Four Different Types of DFDS

Current Logical� Physical aspects of system are removed as much

as possible� Current system is reduced to data and processes

that transform themNew Logical� Includes additional functions� Obsolete functions are removed� Inefficient data flows are reorganized

8.298.29

Four Different Types of DFDS

New Physical� Represents the physical implementation of

the new system

8.308.30

Guidelines for Drawing DFDs

Completeness� DFD must include all components

necessary for system� Each component must be fully described in

the project dictionary or CASE repositoryConsistency� The extent to which information contained

on one level of a set of nested DFDs is also included on other levels

8.318.31

Guidelines for Drawing DFDs

Timing� Time is not represented well on DFDs� Best to draw DFDs as if the system has

never started and will never stop.Iterative Development� Analyst should expect to redraw diagram

several times before reaching the closest approximation to the system being modeled

8.328.32

Guidelines for Drawing DFDs

Primitive DFDs� Lowest logical level of decomposition� Decision has to be made when to stop

decomposition

8.338.33

Guidelines for Drawing DFDs

Rules for stopping decomposition� When each process has been reduced to a

single decision, calculation or database operation

� When each data store represents data about a single entity

� When the system user does not care to see any more detail

8.348.34

Guidelines for Drawing DFDs

Rules for stopping decomposition (continued)� When every data flow does not need to be split

further to show that data are handled in various ways

� When you believe that you have shown each business form or transaction, on-line display and report as a single data flow

� When you believe that there is a separate process for each choice on all lowest-level menu options

8.358.35

Using DFDs as Analysis Tools

Gap Analysis� The process of discovering discrepancies

between two or more sets of data flow diagrams or discrepancies within a single DFD

Inefficiencies in a system can often be identified through DFDs

8.368.36

Using DFDs in Business Process Reengineering

Example: IBM Credit� See Figure 8-20 – before reengineering� Credit approval process required six days

before BPR� Figure 8-21 depicts DFD after

reengineering� IBM was able to process 100 times the

number of transactions in the same amount of time

8.378.37

Oracle’s Process Modeler and Functional Hierarchy Diagrams

Process Modeler� Unique to Oracle� Similar to DFDS but outputs and methods differ in

several ways.� Table 8-4 illustrates differences

Functional Hierarchy Diagrams� Picture of various tasks performed in a business

and how they are related� Tasks are broken down into their various parts� Does not include data flows

8.388.38

Summary

Data flow diagrams (DFD)� Symbols� Rules for creating� Decomposition� Balancing

Four different kinds of DFDs� Current Physical� Current Logical� New Logical� New Physical

8.398.39

Summary

DFDs for AnalysisDFDs for Business Process Reengineering (BPR)Oracle’s Process ModelerFunctional Hierarchy Diagrams

8.408.40